It has been considered from the viewpoint of global environment protection to use a fuel cell as a power supply for a motor operable in place of an internal engine for automobiles, to drive an automobile by the motor. The fuel cell does not need the use of a fossil fuel accompanied by the problem of resource depletion, and can be free from emissions such as exhaust gases. The fuel cell further has eminent advantages, such that it is fairly noise-free, and affords the efficiency of energy collection to be higher than other energy machinery.
Fuel cells are categorized in accordance with the kind of electrolyte in use, into a polymer electrolyte type, a phosphate type, a molten carbonate type, a solid oxide type, etc. As one of them, the polymer electrolyte type (PEFC: Polymer Electrolyte Fuel Cell) is a fuel cell that employs as its electrolyte a membrane of electrolyte of a polymer having a proton exchange group in the molecule, making use of the function the polymer electrolyte membrane has as a proton-conductive electrolyte with a saturated content of water. This polymer electrolyte fuel cell works at relatively low temperatures, and with a high efficiency of power generation. Moreover, the polymer electrolyte fuel cell is allowed to be small in size and light in weight, together with other associated equipment, and is expected to have a variety of applications including mounting to electric vehicles.
The above-noted polymer electrolyte fuel cell includes a fuel cell stack. The fuel cell stack is integrally configured as a lamination of a plurality of unit cells each working as a fundamental unit for power generation by electrochemical reactions, the lamination being sandwiched with end flanges put on both ends, and held tightened by tie bolts. The unit cells are each configured with a solid polymer electrolyte membrane, and a combination of an anode (hydrogen electrode) and a cathode (oxygen electrode) joined to both sides thereof.
FIG. 16 shows in a sectional view the configuration of a unit cell constituting a fuel cell stack. As shown in FIG. 16, the unit cell 90 has a membrane electrode assembly, in which a solid polymer electrolyte membrane 91 is integrated with a combination of an oxygen electrode 92 and a hydrogen electrode 93 joined to both sides thereof. The oxygen electrode 92 and the hydrogen electrode 93 have a two-layered structure configured with a reaction film 94 and a gas diffusion layer (GDL) 95, the reaction film 94 contacting the solid polymer electrolyte membrane 91. On both sides of the combination of oxygen electrode 92 and hydrogen electrode 93, an oxygen electrode side separator 96 and a hydrogen electrode side separator 97 are arranged for lamination, respectively. And, by the oxygen electrode side separator 96 and the hydrogen electrode side separator 97, there are defined oxygen as channels, hydrogen gas channels, and cooling water channels.
For manufacture of the unit cell 90 configured as described, the oxygen electrode 92 and the hydrogen electrode 93 are disposed on both sides of the solid polymer electrolyte membrane 91, and integrally joined thereto, typically by a hot pressing method, to form the membrane electrode assembly, and then, the separators 96 and 97 are disposed on both sides of the membrane electrode assembly. The unit cell 90 constitutes a fuel cell, where a gaseous mixture of hydrogen, carbon dioxide, nitrogen, and water vapor is supplied at the side of hydrogen electrode 93, and air with water vapor, at the side of oxygen electrode 92, whereby electrochemical reactions are caused principally at contact surfaces between solid polymer electrolyte membrane 91 and reaction films 94. More specific reactions will be described below.
In the above-noted configuration of unit cell 90, with oxygen gases and hydrogen gases distributed to oxygen gas channels and hydrogen gas channels, respectively, oxygen gases and hydrogen gases are supplied through gas diffusion layers 95 to the reaction films 94, causing the following reactions in the reaction films 94.Hydrogen electrode side:H2→2H++2e−  formula (1)Oxygen electrode side:(½)O2+2H++2e−→H2O  formula (2)
At the side of hydrogen electrode 93 with hydrogen gas supplied, the reaction of formula (1) proceeds, producing H+ and e−. H+ is hydrated, which moves in the solid polymer electrolyte membrane 91, flowing toward the side of hydrogen electrode 93, while e− is conducted through a load 98, flowing from the hydrogen electrode 93 to the oxygen electrode 93. At the side of oxygen electrode 92 with H+ and e− and oxygen gas supplied, the reaction of formula (2) proceeds, generating electric power.
For fuel cells, separators should have a function of electrical connection between unit cells, as described, and need a good conductivity of electricity, and low contact resistances to component materials of gas diffusion layers and the like. Moreover, an electrolyte membrane of solid polymer type, made of a polymer with multiple sulfonate groups, is humidified to employ sulfonate groups for proton exchange, so as to be proton-conductive. For the electrolyte membrane of solid polymer type, which is weakly acidic, the fuel cell separators are required to be corrosive-resistant against sulfate acidities around pH2 to pH3. Still more, for fuel cells, gases to be supplied have temperatures as hot as within 80° C. to 90° C., and it has not simply the hydrogen electrode, where H+ is produced, but also the oxygen electrode, where oxygen as well as air or the like passes, constituting an oxidizing environment with imposition of potentials around 0.6 V to 1 V vs. SHE relative to a standard hydrogen electrode potential. Hence, for the oxygen electrode, as well as for the hydrogen electrode, the fuel cell separator is required to have a corrosion resistance endurable under a strong acidic atmosphere. It is noted that the corrosion resistance now required means a durability that permits the fuel cell separator to have a maintained performance of electric conduction even under a strong acidic environment. In other words, as cations are transferred into humidifying water or production water due to the reaction of formula (2), they are bonded with those sulfonate groups which inherently should have made ways for protons, and thus occupy the sulfonate groups, constituting an environment that deteriorates a power generating characteristic of the electrolyte membrane, where the corrosion resistance should be measured.
To this point, for separators for fuel cells, attempts have been made to employ an electrically well conductive and excellently corrosion-resistive stainless steel or titanium material such as a pure titanium material for industrial use. The stainless steel has a dense passive film formed on the surface with oxides or hydroxides containing chromium as a principal metallic element, hydrates of them, or the like. Likewise, the titanium material has a dense passive film formed on the surface with titanium oxides or titanium hydroxides, hydrates of them, or the like. The stainless steel as well as the titanium material is thus well anti-corrosive.
However, the above-noted passive films have contact resistances to a carbon paper employed typically as a gas diffusion layer. Fuel cells have an over-voltage due to a resistance polarization therein, although for stationary applications affording a waste heat collection, such as by co-generation, the heat efficiency can be enhanced as a total. But, for applications to automobiles, where heat losses due to contact resistances have to be simply wasted outside, through cooling water, from a radiator, the efficiency of power generation is to decrease, as the contact resistances have an increased influence. Further, the decrease in efficiency of power generation is equivalent to an increase in heat dissipation, which leads to the need for installation of an enlarged cooling system, with a greater influence of contact resistances, as an important issue to be solved.
Although fuel cells have a theoretical voltage, which is 1.23 V per unit cell, the voltage that can be actually taken out is dropped due to reaction polarization, gas diffusion polarization, and resistance polarization, and the voltage decreases, as the current to be taken out increases. Further, in applications to automobiles, where increasing power density per unit volume or unit weight is wanted, the service tends to have a greater current density than for stationary use, e.g., a current density of 1 A/cm2. For the current density of 1 A/cm2, if the contact resistance between separator and carbon paper is kept within a range of 40 mΩ·cm2 or less, the efficiency reduction due to contact resistance is considered as controllable.
In this respect, there is proposed a separator for fuel cells in Japanese Patent Application Laying-Open Publication No. 10-228914, in which a stainless steel is press-formed, and thereafter, a gold skin is formed directly on the surface to be brought into contact with an electrode. Further, there is proposed a separator for fuel cells in Japanese Patent Application Laying-Open Publication No. 2001-6713, in which a stainless steel is molded in the form of a separator for fuel cells, and thereafter, for the surfaces that will have contact resistances when brought into contact with an electrode, their passive films are removed, and a precious metal or a precious metal alloy is attached.